1. Field of the Invention
The present invention relates to a fuel control apparatus for an internal-combustion engine (hereinafter, referred to simply as "engine") and, more particularly, to a fuel control apparatus for an engine provided at least with an exhaust gas recirculation (hereinafter abbreviated to "EGR") equipment for supplying the exhaust gas to the intake system of the engine.
2. Discussion of Background
A conventional EGR equipment of an exhaust pressure control system mounted on an engine comprises an EGR control valve for controlling the exhaust gas recirculating ratio, and an exhaust gas transducer for operating the EGR control valve. The exhaust pressure transducer is operated by the exhaust pressure, which increases with the load on the engine. The exhaust pressure transducer opens a passage when the exhaust pressure decreases to introduce air into the EGR control valve. Thus, the opening of the EGR control valve is decreased with decrease in the load on the engine. Accordingly, the exhaust gas-recirculation ration (EGR ratio) K.sub.EGR (the ratio of the flow rate of the recirculated exhaust gas to the flow rate of intake air) can be maintained on a maximum level over a comparatively wide range of load, i.e., a comparatively wide range of the intake pipe pressure as indicated by solid lines in FIGS. 6(A) and 6(B).
A bypass air control valve is provided in a bypass intake passage bypassing a throttle valve to regulate the sectional area of the opening. The bypass air control valve controls the flow rate of air flowing through the bypass intake passage according to the operating mode of the air conditioner during idling, according to the variation of the throttle opening when the dashpot is necessary, and according to the deviation of the actual engine speed from a desired engine speed for controlling the idling speed.
The operation of a fuel control apparatus applied to such an engine will be described hereinafter with reference to FIG. 8. In steps S1, S2 and S3, engine speed, N.sub.E, intake pipe pressure PB and throttle opening .THETA. are detected sequentially and data N.sub.e, Pb and .theta. engine speed N.sub.E, intake pipe pressure PB and throttle opening .THETA. are read. In step S4, a bypass air flow rate to be set by the bypass air control valve is calculated taking into consideration data representing the operating mode of the engine including engine speed N.sub.E, throttle opening .THETA. and the status of the switch of the air conditioner. In step S5, volumetric efficiency C.sub.EVW (N.sub.e, Pb) during the operation of the EGR system is determined by mapping a two-dimensional map indexed by the engine speed date N.sub.e and the intake pipe pressure data Pb. In step S6, a fuel injection valve driving time .tau. is calculated by using a formula: .tau.=K.times.Pb.times.C.sub.EVW (N.sub.e, Pb), where K is a constant. After step S6 has been completed, the operation returns to step S1, and the the foregoing steps are repeated.
Suppose that the EGR ratio for the engine speed data N.sub.e and the intake pipe pressure data Pb is K.sub.EGR (N.sub.e, Pb), and the volumetric efficiency of the engine while the exhaust gas is not recirculated is C.sub.EVWO (N.sub.e, Pb). Then, the volumetric efficiency C.sub.EVW (N.sub.e, Pb) of the engine while the exhaust gas is recirculated is expressed by: C.sub.EVW =C.sub.EVWO .times.(1-K.sub.EGR). Accordingly, The C.sub.EVW can be determined beforehand on the basis of K.sub.EGR and C.sub.EVWO determined experimentally for the engine speed data N.sub.e and the intake pipe pressure data Pb.
The conventional fuel control apparatus for an engine regulates the quantity of fuel to be injected according to the EGR ratio K.sub.EGR by using the volumetric efficiency C.sub.EVW for an operating state in which the exhaust gas is recirculated. However, since the EGR ratio K.sub.EGR is determined on an assumption that the atmospheric pressure is 1 atm and the bypass passage is closed, the volumetric efficiency C.sub.EVW, which is dependent on the EGR ratio K.sub.EGR and is used for determining the fuel injection quantity, is determined without taking into consideration the variation of bypass air flow rate and that of the atmospheric pressure.
FIG. 6(A) is a graph showing the variation of EGR ratio K.sub.EGR with the intake pipe pressure data Pb for an operating mode in which the bypass passage is open (broken line) and an operating mode in which the bypass passage is closed (solid line) when the atmospheric pressure and the engine speed N.sub.E are fixed. Since the throttle opening decreases as the bypass air flow rate increases when the intake pipe pressure is fixed, the actual EGR ratio K.sub.EGR decreases, namely, the actual volumetric efficiency increases, so that air-fuel ratio increases, namely, the air-fuel mixture becomes lean.
FIG. 6(B) is a graph showing the variation of the EGR ratio K.sub.EGR with the intake pipe pressure data Pb for a low atmospheric pressure (alternate long and short dash line) and a high atmospheric pressure (solid line) when the engine speed N.sub.E is fixed. When the intake pipe pressure is fixed, the throttle opening under a low atmospheric pressure is greater than that under a high atmospheric pressure, and hence the actual EGR ratio K.sub.EGR increases, namely, the actual volumetric efficiency decreases. Accordingly, air-fuel ratio decreases, namely, the air-fuel mixture becomes rich under a relatively low atmospheric pressure.
The variation of the air-fuel ratio according to the variation of the bypass air flow rate or the variation of the atmospheric pressure deteriorates the quality of the exhaust gas and the performance of the engine.
The fuel control system for the engine equipped with such an EGR equipment of an exhaust pressure control apparatus executes fuel injection control operation as shown in FIG. 9. In FIG. 9, steps S21 to S24 are the same as steps S1 to S4 of FIG. 8 and hence the description thereof will be omitted.
In step S25, a volumetric efficiency C.sub.EV (N.sub.e, Pb) for the engine speed data N.sub.e and the intake pipe pressure data Pb for a state in which the exhaust gas is not recirculated is determined by mapping a two-dimensional map. In step S26, an EGR ratio K.sub.EGR (N.sub.e, Pb) for the engine speed data N.sub.e and the intake pipe pressure data Pb is determined by mapping. In step S27, a fuel injection valve driving time .tau.=K.times.Pb.times.C.sub.EV (N.sub.e, Pb).times.{1-K.sub.EGR (N.sub.e, Pb)}, where K is a constant. After the completion of step S27, he operation returns to step S21 to repeat these steps. Thus, this conventional fuel control apparatus regulates fuel injection quantity through the regulation of fuel injection valve driving time on the basis of the EGR ration K.sub.EGR in a state where the atmospheric pressure is 1 atm, and the engine speed and the intake pipe pressure in a state where the bypass passage is closed, and does not take into consideration bypass air flow rate and the variation of the atmospheric pressure.